One that is flat on one side and bulges out on the other is called a _convex lens_; if it bulges out on both sides it is a _double convex lens_; if it is hollowed in on one side and flat on the other it is a _concave lens_; if hollowed in on both sides we call it a _double concave lens_; and when it is hollowed in on one side and bulged out on the other, as any watch crystal does, it is a _concave convex lens_." "Where did you learn all that, Sam ?" asked Tom.
"I learned part of it with father's spectacles, and part out of a book father lent me when I asked him why I couldn't make the bright, hot spot with a pair of near-sighted glasses that I found in one of mother's old work boxes.

You see, when people begin to get old, their eyes flatten a little, and so everything they look at seems to be shaved off.

They see well enough at a distance, but can't see small things close to them." "Is that the reason pa always looks over his spectacles when he looks at me ?" asked Judie.
"Yes, little woman.

He can't see to read without his glasses, but he can see you across the room without them, well enough.

Well, to remedy this defect, old people wear spectacles with double convex lenses in them.